GENERAL MICROBIOLOGY - T.P. Pirog - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION, AND TRANSFER OF TRAITS

19.3. TRANSFER OF TRAITS AND GENETIC RECOMBINATION

19.3.3. Transduction

Transduction is The process of DNA transfer from a donor Cell to a recipient cell mediated by Bacteriophages. Typically, the phage carries only a small fragment of the host DNA. There are two types of transduction: generalized transduction, in which any fragment of the host DNA can be transferred, and specialized transduction, in which only specific DNA fragments are transferred. During transduction, phages often lose The ability to lyse host Cells.

Generalized transduction. The host cell DNA is incorporated into the phage particle either In addition to or instead of its own genome. The transfer of bacterial chromosome segments by phages was discovered by J. Lederberg and N.Z. Zinder in Salmonella typhimurium. Donor strain B was infected with bacteriophage P22. Following lysis of the host cell, free phages were isolated and incubated with cells of recipient strain B . After plating the incubated cells onto an appropriate medium, recombinants appeared that exhibited the characteristics of donor strain B .

Specialized transduction. Certain phage genes are replaced by host genes. The most well-known is the transduction mediated by phage lambda, which transduces only specific genes, namely the gal and bio genes. Upon transitioning to the prophage state, this phage integrates into a specific site on the bacterial host chromosome—between the gal and bio genes. The excision of the phage DNA from the bacterial chromosome (e.g., upon UV irradiation) may occur improperly, meaning that a portion of the phage DNA remains in the chromosome while adjacent host cell genes are packaged into the phage DNA. When cells defective in a particular Gene (such as gal) are infected with such a transducing phage, recombination can occur, leading to the replacement of the defective bacterial gene with the intact transducing gene. This results in The formation of gal recombinants (transductants).

Similarly, genes are transferred by bacteriophage Phi80. Its DNA integrates into the chromosome near the genes encoding Enzymes responsible for Tryptophan Biosynthesis. For this reason, bacteriophage Phi80 is particularly well-suited for the transfer of trp genes.

A prerequisite for successful gene transfer in specialized transduction (unlike generalized transduction) is the Integration of the phage into the host cell genome.